Crystal filter system



` March 1, 1949. P. H. CRAIG 2,463,249

CRYSTAL FILTER SYSTEM Filed June 14, 1946 Rec# iev Redcance y v dh 5/ 4 g Buffer ampli/fer m 736e E Z/V VE.' N TOR Patented Mar. l, 1949 UNITED CRYSTAL FILTER SYSTEM Palmer H. Craig, Gainesville, Fla., assigner to Invex incorporated, a corporation of Florida Application June 14, 1946, Serial No. 676,597

8 Claims. l

This invention relates to crystal filter systems for use in various signalling circuits or wave transmission circuits, and it is especially concerned with arrangements for increasing the effectiveness of the iilter in diierentiating between slightly different frequencies which follow each other in very rapid succession.

My lter system is concerned with arrangements for exciting the crystals of the iilter to secure quick response to the impressed. signal Wave, and also with arrangements for damping the crystals and preventing continued vibration thereof after the signal Wave has ceased to act on the crystals.

Crystal filters are very useful in applications where high selectivity is required, but one drawback to this type of filter is that it continues to vibrate for a time after the energizing signal has ceased to act on the crystal. In most uses of crystal filters, this persistence of vibration is not objectionable, but in certain applications, such as in television systems, it may limit the speed of operation of the system. For example, in television systems of the type disclosed in my copending application Serial No. 459,705, filed September 25, 1942, (now Patent No. 2,444,221) it is necessary for the filter to be highly selective and at the same time to respond quickly to sudden changes in the value of signal. This is also true in systems of the type disclosed in my Patent 2,393,890. The main object of my invention is to provide arrangements for damping the oscillation of the filter after the signal wave has ceased to act on the crystal.

W'hile my invention will be described in connection with a receiving arrangement in a television system, it will be obvious that it is not limited to this application but may be applied to other fields.

One embodiment of my invention is illustrated in the accompanying drawing in which Figure l is a diagrammatic representation of a television receiving arrangement having my lter system embodied therein;

Figure 2 is a series of wave curves for illustrating the operation of the filter system; and

Figure 3 is a curve showing a diierent wave form which may be employed if desired.

Referring to the drawing, I have illustrated my crystal' filter system embodied in a receiving arrangement for receiving television signals of the type transmitted in the system disclosed in my copending application 459,705. In arrangements of this type, a composite wave is radiated and is formed of component waves each of a frequency dependent upon the position of the correspond'- ing elementary area in the image area, and the amplitude of each component wave is dependent upon the degree of illumination of the corresponding elementary area. At the receiving station, the composite wave is received and is heterodyned with a locally generated wave which is varied in frequency periodically and in a manner to produce a beat wave of predetermined frequency with each component wave in succession. The beat wave is passed through the crystal filter and is then de-modulated and applied to a cathode ray tube.

The antenna I is connected to a heterodyne receiver 2 the output of which is connected to the input of a variable gain amplifier 3, and the output of this amplifier is connected to a crystal iilter, preferably of the lattice type involving crystals 4 and 5 connected as shown. The output of the crystal lter is connected to the input of the demodulator 6 the output ci which is connected to the cathode la and the grid 'lb of a cathode ray tube l. The crystal filter 4-5 is highly selective and is designed to pass a wave of predetermined frequency which results from the heterodyning of the incoming signal wave with a locally generated Wave supplied to receiver 2 over the connection 2a. As will be explained later, the locally generated wave is varied periodically throughout a frequency range such that each component of the incoming Wave will produce a beat wave oi a frequency to pass the filter 4 5.

To carry out the objects of my inventioInI provide la pulse generator 8 which may assume various forms for generating a control wave having a lower frequency than the beat wave. For example, if the curve (a) in Figure 2 represents the beat wave, then one suitable wave form for the pulse generated by generator 8' is shown in curve (b), although other wave forms may be employed as will be explained later. Suitable arrangements for the generator 8 are already known and examples will be found in various publications such as pages 1'76 to 190, 1st edition, of Ultra-High Frequency Techniques by Brainerd, Koehler, Reich and Woodruff, page 186 of Time Bases by O. S. Puckle and pages to 162, vol. 3, of Electronics Engineering Manual, McGraw-Hill Bock Company. A connection '8a from the output of receiver'2 may be provided for synchronizing the operation of pulse generator 8. Preferably the generator 8 is designed to produce a pulse frequency equal to the elementary area scanning frequency, that is, the generator de,

3 velcps one complete cycle of control wave for each scanning period of an elementary area.

A control wave having the form shown in Figure 2b is supplied to the input amplifier 9 which includes tube 9a normally biased to cutoff by a source of biasing potential 9b. The plate circuit of this tube is completed through a series resistance 9c and inductance 9d and a source of plate current 9e. The potential developed across resistance 9c is applied to the amplifier 3 to vary the gain of this amplifier in a well known manner. Since tube 9a is normally biased to cutoff, there will be no potential drop across resistance 9c, and there will be a maximum gain in amplifier 3. During the time when a positive pulse is being applied to the tube 9a from generator 8, current will fiow in the circuit of this tube and through resistance 9c. Due to the presence of the inductance 9d, this current will start from zero and will increase exponentially with time. As the current flowing through 9c increases, the control potential supplied from this resistance to the amplifier 3 reduces the amount of gain of the amplifier as the potential increases, and the net result is that the crystals 4 and 5 in the lter lattice are excited with strong excitation at the beginning of each signal period and the value of the applied signal is decreased towards the end of the signal period. This serves to rapidly set the crystals into oscillation at the beginning of each signal period. l The wave applied to the input of the crystal lter during the positive pulse of. the control wave is shown in Figure 2c, and it will be seen that the amplitude of the wave is at a maximum at the beginning of the positive control pulse and decreases towards the end.

Each negative pulse from the control generator 8 has no effect on tube 9a or upon variable gain amplifier 3, but the negative pulse is transmitted by the rectifier I0 to the tuned circuit II Which is set into oscillation at a frequency corresponding to the frequency of the crystal filter. This oscillation is amplified by buffer I2 and is'applied to the input terminals of the filter in phase 0pposition to the signal Wave applied from amplifier 3. Also, the wave transmitted from tuned circuit II is amplified by the buffer I2 to completely cancel out the wave transmitted by amplifier 3 from receiver 2 and applies a Wave of opposite v phase to the input of the filter. This is indicated in Figure 2c by the fact that the first pulse of the curve I2a applied to the lter from buffer I2 is in the opposite direction from the pulse 3a. which is supplied from the amplifier 3 and is shown in dotted lines in Figure 2. Since the wave supplied from buffer I2 greatly exceeds the amplitude 0f the wave from amplifier 3, the Wave from buffer I2 serves to damp the oscillation of the crystal, and the resultant wave applied to the crystal filter during the negative pulse of the control wave is shown in Figure 2c where it will be seen that the amplitude of the resultant wave gradually reduces to zero, reverses in phase, and increases again to a maximum value at the beginning of the next positive pulse of the control wave. It will be understood that the oscillation from the tuned circuit II does not persist longer than the duration of the negative pulse of the control wave, and the length of this oscillation may be controlled by a damping resistor Ila connected in shunt to the circuit.

low value, and instead of using the amplifier 9 to reduce the gain of the amplifier 3, the control Wave from generator 8 may be applied directly to the gain control terminals of amplifier 3 in such manner that the positive pulse of the control wave increases the gain of the amplifier 3 while the positive pulse is applied. The control wave may have the wave form shown in Figure 2b, but if it is desired to have the crystal lter strongly excited at the beginning of the positive pulse, a Wave form like that shown in Figure 3 may be employed. Also, it will be noted that the negative pulse of the control Wave may be made shorter than the positive pulse as shown in Figure 3.

From the foregoing it will be seen that the pulse generator 8 serves as a periodic switching device for alternately applying a bucking wave to the crystal lter to damp the filter during onehalf of each cycle of the control generator. During the remaining half of each cycle of the control wave, the filter is free to operate under the influence of the signal to be transmitted, and the arrangement is such that the initial amplitude of the applied signal is at a maximum and de creases during the transmission period.

The pulse generator 8 may also be employed for the purpose of supplying the locallygenerated oscillation for receiver 2 and for supplying the scanning voltages for the cathode ray tube 1. For this purpose, current from generator 8 is supplied to rectifier I3 which may also amplify the rectified pulses. The pulses from rectifier I3 are supplied to an integrating circuit I4 of any suitable construction such as a series resistor Ida, and a shunt condenser Idb which is charged to a progressively increasing voltage by the pulses supplied from rectifier I3. When the voltage cross condenser I4b reaches a predetermined value, the condenser is discharged by a suitable form of automatic switch such as the gaseous discharge tube I4@ connected in shunt to the condenser. This arrangement results in the generation of a saWtooth voltage wave which is applied to a reactance tube I5 Which varies the frequency of an oscillator I6. The output of oscillator I6 is supplied to the receiver 2 through a buffer amplifier I1 and connection 2a. The arrangement is such that the frequency of the oscillator I6 is varied progressively throughout a range of frequencies which will combine with each component frequency received by antenna I and produce a beat Wave of a frequency to pass through the crystal filter.

Since each sawtooth wave generated by the integrating device I4 produces a complete scanning cycle, the output Voltage of this integrating device may be employed for producing the vertical scanning in cathode ray tube l', and this is accomplished by supplying current from the output of the integrating device I4 to a buffer amplifier I8 which supplies the deflecting potential to the vertical plates 1d. l

The horizontal scanning may be accomplished by the same type of integrating circuit but it must have a frequency higher than the vertical scanning. The horizontal scanning potential may be obtained by an integrating device I9 similar to the device I 4 and involving a series resistor ISa, a shunt condenser IQb and a dis,-

,charging device |90. The input of the integrator I9 is connected to the output of rectifier I3, and the output of this device is applied to the horizontal deiiecting plates 'Ie after being amplified by the buffer 20. l Y l From the foregoing it will be understood that in the Wave transmission system of my invention, the crystal filter responds more accurately and quickly to the signal waves which follow each other in rapid succession, and this result is accomplished by providing .for the damping of the crystal after each signal wave excitation. While I prefer to apply an opposing wave of the same frequency as the signal wave to the input of the crystal during the negative pulse of the control wave, the crystal may be damped in other ways. For example, the negative pulse from generator may be employed to connect a shunting circuit across the filter during the negative pulse, and one suitable arrangement would be to provide a vacuum tube normally biased to cut-off and having its plate and cathode elements connected across the input of the filter through a blockingr condenser, and the negative pulse from generator 8 would control a phase reversing amplifier to supply a positive potential to the grid of the shorting tube during the time of the negative pulse. The low impedance path established by the shorting tube would act to damp the crystal during the negative pulse of the generator 8.

It will be understood that instead of connecting; the damping circuit to the input terminals of the iilter, this circuit may be connected to a separate set of plates provided on the filter for this purpose, and this applies both to the arrangement shown in the drawing and to the arrangement involving the shorting tube.

It may be noted that the waves of Figures 2a and 2c would ordinarily be sinusoidal, but they are shown of peaked shape for simplicity.

I claim:

1. A wave transmission system comprising a crystal filter adapted to pass a predetermined frequency, a source of signal wave connected to the input of said lter, a source of control wave of lower frequency than said signal wave, and means controlled by one half-cycle pulse of said control wave for supplying to the input of said filter a Wave of the same frequency as said signal wave but of opposite phase.

2. A wave transmission system comprising, in combination, a crystal filter adapted to pass a predetermined frequency, a source of signal wave, a source of control Wave of relatively low frequency with respect to said predetermined frequency, means controlled by one half-cycle pulse of said control Wave for supplying signal waves from said source to said filter, and means controlled by the other half-cycle pulse of said control wave for supplying to the input of said filter a wave for damping the oscillation of said filter.

3. A wave transmission system comprising a crystal filterl adapted to pass a predetermined frequency, a source of signal wave connected to the input of said filter, a source of control wave of lower frequency than said signal Wave, and means controlled by one half-cycle pulse of said control wave for damping the oscillation of said lter.

4. A Wave transmission system comprising, in combination, a crystal filter adapted to pass a predetermined frequency, a source of signal wave, a source of control wave of relatively low frequency with respect to said predetermined frequency, means controlled by one half-cycle pulse of said control wave for supplying signal waves from said source to said filter, and means controlled by the other half-cycle pulse of said control wave for damping the oscillation of said filter.

5. A wave transmission system comprising, in combination, a crystal filter adapted to pass a predetermined frequency, a source of signal wave, a variable gain amplifier connecting said source to the input of said lter and being normally adjusted for maximum gain, a source of control wave of relatively low frequency with respect to said predetermined frequency, means controlled by one half-cycle pulse of said control wave for progressively reducing the gain of said amplifier during said pulse and means controlled by the other half-cycle pulse of said control wave for damping the oscillation of said filter.

6. A wave transmission system comprising, in combination, a crystal filter adapted to pass a predetermined frequency, a source of signal wave, a variable gain amplifier connecting said source to the input of said filter and being normally adjusted for low gain, a source of control wave of relatively low frequency with respect to said predetermined frequency, means controlled by one half-cycle pulse of said control wave for increasing the gain of said amplifier during said pulse, and means controlled by the other half-cycle pulse of said control wave for damping the oscillation of said filter.

'7. A wave transmission system according to claim 6 wherein said gain control means operates to establish a relatively high gain at the beginning of the control pulse and the gain decreases towards the end of said control pulse.

8. A wave transmission system comprising, in combination, a crystal filter adapted to pass a predetermined frequency, a source of signal wave, a variable gain amplifier for connecting said source to the input of said lter, a generator for producing a control Wave of lower frequency than said predetermined frequency, means controlled by said signal wave for synchronizing the operation of said generator, a second generator for producing a wave of a frequency corresponding to said predetermined frequency but of opposite phase relation with respect to said signal wave, means controlled by one half-cycle pulse of said control wave for varying the gain of said amplifier so that the gain is the maximum at the beginning of -said pulse and decreases towards the end thereof, and means controlled by the other halfcycle pulse of said control wave for supplying to the input of said filter waves from said second generator to damp the oscillation of said filter.

PALMER H. CRAIG.

No references cited. 

